How long will you be away for? If you are travelling for a while, a kindle would be a great idea.

We spent some time in Europe when DD1 was 10. The one thing she missed was books. We had taken a few, but she is an avid reader and had finished them by the time the plane trip was over, leaving her nothing new to read while we were away.
It was not easy finding children's books in English in France.

A kindle means you can take thousands, with very little extra weight to carry.

I would get the kindle so she can read on planes/trains without the hassle of lugging space hogging books everywhere. I wish I'd bought one for DS11 before we went o/s last year. He ran out of books to read part way through, we ended up buying more from English bookshops in Italy.

A good quality pair of headphones are very useful too, if she doesn't have a pair already.

Oh, DS had his own camera too (<$100) which was great. It was interesting to see what he focused on and found interesting compared to what we chose to photograph.

Could you do a voucher for a souvenir that you would normally say no too? A trinket box, or poster or frame or something that would normally be out of the "budget" or for what ever reason not allowed. You could still put restrictions (like up to a certain amount or that it needs to fit in a certain space), but there would still be something tangible to open.

A Garmin? Not the kid’s one, a “proper” one. We all had ours on our O/S trip last year and our kids loved seeing how many steps/distance they covered each day.
Noise-cancelling headphones were a massive hit for our kids as well.

In the end, I bought DD a Matildas jersey and new warm socks for her birthday present.

We went to Algonquin the day after her birthday so for her bday we spent the day at Niagara Falls and then went back to the big shopping centre in Toronto where she got to buy a new book from Indigo books and was treated to ice cream afterwards.

She has declared that it was the best birthday ever.
Thanks for the input.